1. Field of the Invention
Devices, systems, and methods consistent with the invention relate to a polarization-maintaining optical fiber which is suitable for production of optical fiber gyros that are used on moving bodies, such as aircraft, for precise attitude and position controls. More specifically, the present invention relates to a polarization-maintaining optical fiber whose performance does not easily deteriorate when bent with a small bending radius and has a stabilized performance, and to an optical fiber gyro using the same.
2. Description of the Related Art
Polarization-maintaining optical fibers are single-mode optical waveguides (single-mode fibers) which can implement optical transmission while maintaining linear polarization. FIG. 1 illustrates a typical structure thereof. A polarization-maintaining optical fiber 1 is formed by a central core 2, a pair of stress-applying parts 3a and 3b disposed at opposed sides with respect to the core 2, and a cladding 4 which surrounds the core 2 and the stress-applying parts 3a and 3b. The stress-applying parts 3a and 3b of the polarization-maintaining optical fiber 1 are a pair of glass areas used to induce a birefringence in the core 2, extending in a lengthwise direction, and which are separately provided within the cladding 4 and located in diametrically opposed positions. One heretofore known type thereof is characterized in that a thermal expansion coefficient of the stress-applying parts 3a and 3b is different from that of the cladding 4 and that a dimension in cross-section of at least one of the stress-applying parts 3a and 3b is larger than a diameter of the core 2. A polarization-maintaining optical fiber including such a structure is called a PANDA (Polarization-maintaining and absorption-reducing optical fiber).
Polarization-maintaining optical fibers have been conventionally employed as optical fibers for coupling optical components which exhibit polarization dependence. In addition, the polarization-maintaining optical fibers themselves are manufactured so as to form optical transmission components, e.g., fiber gratings and optical fiber couplers, such that they are used in optical transmission devices and various types of measuring equipment, e.g., optical fiber gyroscopes (see, Japanese Unexamined Patent Application, First Publication No. 63106519).
Each of the conventional polarization-maintaining optical fibers generally has a structure as shown in FIG. 1, including a cladding whose diameter (hereinafter referred to as simply “cladding diameter”) is 125 μm, and since such fibers are easy to manufacture, they can be manufactured with uniform quality, and can be easily coupled to conventional silica-based optical waveguide fibers.
Such polarization-maintaining optical fibers having the foregoing characteristics are put to a proof-test. Among spools having a diameter of 60 mm through 40 mm, each of the fibers tested are wound onto a spool having a diameter corresponding to the result of the proof-test. Thereafter, they are incorporated or mounted in optical transmission devices and various types of measuring equipment. Such spool diameters are set in consideration of possible effects of amounts of deformation applied to claddings on the reliability and effects on optical fiber characteristics due to bending.
In an optical fiber gyroscope, e.g., a polarization-maintaining optical fiber in which the fiber is to be wound around a small-diameter spool or with a small bending radius, if the fiber has a cladding 125 μm in diameter as usual, problems arise in which deformation causes strength reduction, thus resulting in deteriorated reliability. To respond to this, a polarization-maintaining optical fiber with a small cladding diameter of 80 μm has been developed (Japanese Unexamined Patent Application, First Publication No. 2003-337238, Fujikura Technical Review, No. 85, p. 1 to p. 9 (published on October 1993), and http://www.fibercore.com/06hb-hbg.php (homepage of Fibercore Limited)).
The above-described Fujikura Technical Review discloses a polarization-maintaining optical fiber in which a cladding is made to a minute diameter of 80 μm, for use in gyroscopes. This polarization-maintaining optical fiber for gyroscopes is structured such that, in order to reduce microbend loss, the fiber has a larger relative refractive index difference Δ between a core and a cladding (hereinafter referred to simply as “relative refractive index difference”) than that of a conventional polarization-maintaining optical fiber, and that the fiber has a smaller mode field diameter (hereinafter referred to as “MFD”).
However, as described above, if an MFD is made smaller, there arises the disadvantage that splice loss tends to increase due to an axial offset incurred when implementing fusion-splicing.
Japanese Unexamined Patent Application, First Publication No. 2003-337238 discloses fiber parameters which are optimized and designed to solve the above problem. More specifically, this patent publication discloses a polarization-maintaining optical fiber which includes a core, a pair of stress-applying parts provided on opposed sides with respect to the core, and a cladding which surrounds the core and the stress-applying parts, wherein a diameter of the cladding is between 70 μm and 90 μm, a diameter of the stress-applying parts is between 21 μm and 32 μm, a distance between the stress-applying parts is between 6 μm and 17 μm, and a relative refractive index difference (between the core and the cladding) is between 0.3% and 0.5%.
Further, Fibercore Limited offers fibers for fiber optic gyroscopes. The specifications thereof are as follows: cut-off wavelength is between 680 nm and 780 nm, and a diameter of a cladding (of a fiber having an MFD 4.2 μm) is 80 μm. According to their brochure, when 500 m of the fiber is wound around a spool whose outside diameter is 40 mm, −27 dB in polarization crosstalk is obtained. However, when the fiber is wound by a further longer length around a further smaller diameter spool, polarization crosstalk is deteriorated.